Graded changes in light intensity at dawn and dusk are a hallmark of the natural daily illumination cycle, yet their contribution to circadian entrainment remains largely unknown. The research program described below will make use of recently developed twilight simulation technology, in a systematic investigation of the role of twilight transitions in the entrainment of circadian rhythms in nocturnal and diurnal fossorial rodents. In keeping with our long-term goal of elucidating the process of entrainment under naturalistic, ecologically meaningful conditions, hamsters and degus will be provided with a light-excluding nest box and will thus have the opportunity to self-select their daily light exposure. Experiments are designed to compare the effects of light-dark (LD) cycles with and without simulated twilights, and in the presence and absence of dark nest boxes. The aims of the project are (1) to determine whether twilights increase the strength of the LD zeitgeber, by comparing the limits of entrainment under LD cycles with abrupt and simulated twilight transitions; (2) to examine entrainment, light exposure patterns, and photoperiodic effects on the reproductive system under twilight cycles with photoperiod and twilight durations appropriate to different seasons of the year, and under corresponding rectangular LD cycles; (3) to determine the effects of daytime light intensity levels on entrainment and light self-exposure patterns; and (4) to examine entrainment, daily light exposure, and photoperiodic effects across an entire year under light cycles with gradually changing photoperiods and twilight durations, simulating the natural seasonal progression of these parameters. Recent demonstrations of entrainment and resetting of human circadian rhythms by timed exposure to bright light are increasingly finding applications in the treatment of a variety of sleep disorders linked to abnormalities in circadian entrainment. Observations in winter-depressed patients suggest that low intensity dawn twilight signals can have marked therapeutic consequences, and elicit shifts in sleep phase and in melatonin secretion patterns previously thought to require bright light interventions. Thus, a systematic account of twilight-induced effects on circadian entrainment may provide the basis for refined clinical treatments of sleep phase and seasonal affective disorder.